1. Field of the Invention
The invention is generally related to nuclear reactors and more particularly to nuclear reactor control mechanisms.
2. General Background
In nuclear reactors, control mechanisms are required to prevent excessive reactivity, to maintain specific levels of reactivity during operations, and to provide power shaping within the core for the most efficient operation. A second independent control mechanism is usually employed for emergency shutdown. Control mechanisms may be put into four general categories depending upon their basic operating principle: 1) insertion/removal of a fissile, poison, or moderator material into or from the active core region; 2) reflection of leaking neutrons back into the system by variable reflectors, shutters, or rotating drums; 3) geometry changes as in a split core or "godiva" type reactor; and 4) flux shadowing by moving internal parts made of alternating regions of different neutronic materials such as fuel, moderator, or absorber. This movement results in isolation of some core active region from the rest. Patented devices for controlling reactors by the flux shadowing principle of which applicant is aware include the following.
U.S. Pat. No. 2,852,458 discloses coaxial cylinders designed to move rotatably and translatably relative to each other. The cylinders have circumferential sections of material having different neutron absorbing characteristics whereby the movements change the neutron flux shadowing effects.
U.S. Pat. No. 2,898,281 discloses control rods disposed adjacent and parallel to each other with each rod having equal length sections of neutron absorbing and permeable materials and means for longitudinally positioning the rods relative to each other.
U.S. Pat. No. 3,103,479 discloses a control rod formed from different layers of neutron absorbing material.
U.S. Pat. No. 3,347,747 discloses the use of hollow tubes that house neutron absorbing balls wherein the distribution of the balls in the tubes is controlled by fluid flow through the tubes.
U.S. Pat. No. 3,485,717 discloses the use of a cruciform control element formed from tubes having different cross-sectional neutron absorption properties.
U S. Pat. No. 4,707,329 discloses a control rod with inner and outer cylindrical members movable relative to each other and each having alternating poison and nonpoison regions.
Existing control mechanisms are configured in the vertical direction only and most provide only local reactivity changes. No control mechanism exists that divides the core region into halves to provide global power control.